%% @hidden
-module(ra_log_segment_writer).
-behaviour(gen_server).

-export([start_link/1,
         accept_mem_tables/2,
         accept_mem_tables/3,
         truncate_segments/2,
         truncate_segments/3,
         my_segments/1,
         my_segments/2,
         await/0,
         await/1,
         overview/0
        ]).

-export([init/1,
         handle_call/3,
         handle_cast/2,
         handle_info/2,
         terminate/2,
         code_change/3
         ]).

-record(state, {data_dir :: file:filename(),
                segment_conf = #{} :: ra_log_segment:ra_log_segment_options()}).

-include("ra.hrl").

-define(AWAIT_TIMEOUT, 30000).

%%% ra_log_segment_writer
%%% receives a set of closed mem_segments from the wal
%%% appends to the current segment for the ra server
%%% notifies the ra server of any new/updates segments


%%%===================================================================
%%% API functions
%%%===================================================================

start_link(Config) ->
    gen_server:start_link({local, ?MODULE}, ?MODULE, [Config], []).


accept_mem_tables(Tables, WalFile) ->
    accept_mem_tables(?MODULE, Tables, WalFile).

accept_mem_tables(_SegmentWriter, [], undefined) ->
    ok;
accept_mem_tables(SegmentWriter, Tables, WalFile) ->
    gen_server:cast(SegmentWriter, {mem_tables, Tables, WalFile}).

-spec truncate_segments(ra_uid(), ra_log:segment_ref()) -> ok.
truncate_segments(Who, SegRef) ->
    truncate_segments(?MODULE, Who, SegRef).

-spec truncate_segments(atom() | pid(), ra_uid(), ra_log:segment_ref()) -> ok.
truncate_segments(SegWriter, Who, SegRef) ->
    % truncate all closed segment files
    gen_server:cast(SegWriter, {truncate_segments, Who, SegRef}).

%% returns the absolute filenames of all the segments
-spec my_segments(ra_uid()) -> [file:filename()].
my_segments(Who) ->
    my_segments(?MODULE, Who).

-spec my_segments(atom() | pid(), ra_uid()) -> [file:filename()].
my_segments(SegWriter, Who) ->
    gen_server:call(SegWriter, {my_segments, Who}, infinity).

-spec overview() -> #{}.
overview() ->
    gen_server:call(?MODULE, overview).


% used to wait for the segment writer to finish processing anything in flight
await() ->
    await(?MODULE).

await(SegWriter)  ->
    IsAlive = fun IsAlive(undefined) -> false;
                  IsAlive(P) when is_pid(P) ->
                            is_process_alive(P);
                  IsAlive(A) when is_atom(A) ->
                            IsAlive(whereis(A))
              end,
    case IsAlive(SegWriter) of
        true ->
            gen_server:call(SegWriter, await, ?AWAIT_TIMEOUT);
        false ->
            % if it is down it isn't processing anything
            ok
    end.

%%%===================================================================
%%% gen_server callbacks
%%%===================================================================

init([#{data_dir := DataDir} = Conf]) ->
    process_flag(trap_exit, true),
    SegmentConf = maps:get(segment_conf, Conf, #{}),
    {ok, #state{data_dir = DataDir,
                segment_conf = SegmentConf}}.

handle_call(await, _From, State) ->
    {reply, ok, State};
handle_call({my_segments, Who}, _From, State) ->
    SegFiles = segments_for(Who, State),
    {reply, SegFiles, State};
handle_call(overview, _From, State) ->
    {reply, get_overview(State), State}.

segments_for(UId, #state{data_dir = DataDir}) ->
    Dir = filename:join(DataDir, ra_lib:to_list(UId)),
    SegFiles = lists:sort(filelib:wildcard(filename:join(Dir, "*.segment"))),
    SegFiles.

handle_cast({mem_tables, Tables, WalFile}, State0) ->
    State = lists:foldl(fun do_segment/2, State0, Tables),
    % delete wal file once done
    % TODO: test scenario when server crashes after segments but before
    % deleting walfile
    % can we make segment writer idempotent somehow
    ?DEBUG("segment_writer: deleting wal file: ~s~n",
          [filename:basename(WalFile)]),
    %% temporarily disable wal deletion
    %% TODO: this shoudl be a debug option config?
    % Base = filename:basename(WalFile),
    % BkFile = filename:join([State0#state.data_dir, "wals", Base]),
    % filelib:ensure_dir(BkFile),
    % file:copy(WalFile, BkFile),
    _ = file:delete(WalFile),
    %% ensure we release any bin refs that might have been acquired during
    %% segment write
    true = erlang:garbage_collect(),
    {noreply, State};
handle_cast({truncate_segments, Who, {_From, _To, Name} = SegRef},
            #state{segment_conf = SegConf} = State0) ->
    %% remove all segments below the provided SegRef
    %% Also delete the segref if the file hasn't changed
    Files = segments_for(Who, State0),
    {_Keep, Discard} = lists:splitwith(
                         fun (F) ->
                                 ra_lib:to_string(filename:basename(F)) =/= Name
                         end, lists:reverse(Files)),
    case Discard of
        [] ->
            %% should this be possible?
            {noreply, State0};
        [Pivot | Remove] ->
            %% remove all old files
            _ = [_ = file:delete(F) || F <- Remove],
            %% check if the pivot has changed
            {ok, Seg} = ra_log_segment:open(Pivot, #{mode => read}),
            case ra_log_segment:segref(Seg) of
                SegRef ->
                    %% it has not changed - we can delete that too
                    %% as we are deleting the last segment - create an empty
                    %% successor
                    _ = ra_log_segment:close(
                          open_successor_segment(Seg, SegConf)),
                    _ = ra_log_segment:close(Seg),
                    _ = file:delete(Pivot),
                    {noreply, State0};
                _ ->
                    %% the segment has changed - leave it in place
                    _ = ra_log_segment:close(Seg),
                    {noreply, State0}
            end
    end.


handle_info(_Info, State) ->
    {noreply, State}.

terminate(_Reason, _State) ->
    ok.

code_change(_OldVsn, State, _Extra) ->
    {ok, State}.


%%%===================================================================
%%% Internal functions
%%%===================================================================

get_overview(#state{data_dir = Dir,
                    segment_conf = Conf}) ->
    #{data_dir => Dir,
      segment_conf => Conf
     }.

do_segment({ServerUId, StartIdx0, EndIdx, Tid},
           #state{data_dir = DataDir,
                  segment_conf = SegConf} = State) ->
    Dir = filename:join(DataDir, binary_to_list(ServerUId)),

    case open_file(Dir, SegConf) of
        enoent ->
            ?WARN("segment_writer: skipping segment as directory ~s does "
                   "not exist~n", [Dir]),
            %% clean up the tables for this process
            _ = ets:delete(Tid),
            _ = clean_closed_mem_tables(ServerUId, Tid),
            State;
        Segment0 ->
            case append_to_segment(ServerUId, Tid, StartIdx0, EndIdx,
                                   Segment0, SegConf) of
                undefined ->
                    ?WARN("segment_writer: skipping segments for ~w as
                           directory ~s disappeared whilst writing~n",
                           [ServerUId, Dir]),
                    State;
                {Segment1, Closed0} ->
                    % fsync
                    {ok, Segment} = ra_log_segment:sync(Segment1),

                    % notify writerid of new segment update
                    % includes the full range of the segment
                    ClosedSegRefs = [ra_log_segment:segref(S) || S <- Closed0],
                    SegRefs = case ra_log_segment:segref(Segment) of
                                  undefined ->
                                      ClosedSegRefs;
                                  SRef ->
                                      [SRef | ClosedSegRefs]
                              end,

                    _ = ra_log_segment:close(Segment),

                    ok = send_segments(ServerUId, Tid, SegRefs),
                    State
            end
    end.

start_index(ServerUId, StartIdx0) ->
    case ets:lookup(ra_log_snapshot_state, ServerUId) of
        [{_, SnapIdx}] ->
            max(SnapIdx + 1, StartIdx0);
        [] ->
            StartIdx0
    end.

send_segments(ServerUId, Tid, Segments) ->
    Msg = {ra_log_event, {segments, Tid, Segments}},
    case ra_directory:pid_of(ServerUId) of
        undefined ->
            ?DEBUG("ra_log_segment_writer: error sending "
                   "ra_log_event to: "
                   "~s. Error: ~s",
                   [ServerUId, "No Pid"]),
            _ = ets:delete(Tid),
            _ = clean_closed_mem_tables(ServerUId, Tid),
            ok;
        Pid ->
            Pid ! Msg,
            ok
    end.

clean_closed_mem_tables(UId, Tid) ->
    Tables = ets:lookup(ra_log_closed_mem_tables, UId),
    [begin
         ?DEBUG("~w: cleaning closed table for '~s' range: ~b-~b~n",
                [?MODULE, UId, From, To]),
         %% delete the entry in the closed table lookup
         true = ets:delete_object(ra_log_closed_mem_tables, O)
     end || {_, _, From, To, T} = O <- Tables, T == Tid].

append_to_segment(UId, Tid, StartIdx0, EndIdx, Seg, SegConf) ->
    StartIdx = start_index(UId, StartIdx0),
    % EndIdx + 1 because FP
    append_to_segment(UId, Tid, StartIdx, EndIdx+1, Seg, [], SegConf).

append_to_segment(_, _, StartIdx, EndIdx, Seg, Closed, _)
  when StartIdx >= EndIdx ->
    {Seg, Closed};
append_to_segment(UId, Tid, Idx, EndIdx, Seg0, Closed, SegConf) ->
    [{_, Term, Data0}] = ets:lookup(Tid, Idx),
    Data = term_to_binary(Data0),
    case ra_log_segment:append(Seg0, Idx, Term, Data) of
        {ok, Seg} ->
            append_to_segment(UId, Tid, Idx+1, EndIdx, Seg, Closed, SegConf);
        {error, full} ->
            % close and open a new segment
            case open_successor_segment(Seg0, SegConf) of
                undefined ->
                    %% a successor cannot be opened - this is most likely due
                    %% to the directory having been deleted.
                    %% clear close mem tables here
                    _ = ets:delete(Tid),
                    _ = clean_closed_mem_tables(UId, Tid),
                    undefined;
                Seg ->
                    %% re-evaluate snapshot state for the server in case
                    %% a snapshot has completed during segment flush
                    StartIdx = start_index(UId, Idx),
                    % recurse
                    append_to_segment(UId, Tid, StartIdx, EndIdx, Seg,
                                      [Seg0 | Closed], SegConf)
            end
    end.

find_segment_files(Dir) ->
    lists:reverse(
      lists:sort(filelib:wildcard(filename:join(Dir, "*.segment")))).

open_successor_segment(CurSeg, SegConf) ->
    Fn0 = ra_log_segment:filename(CurSeg),
    Fn = ra_lib:zpad_filename_incr(Fn0),
    ok = ra_log_segment:close(CurSeg),
    case ra_log_segment:open(Fn, SegConf) of
        {error, enoent} ->
            %% the directory has been deleted whilst segments were being
            %% written
            undefined;
        {ok, Seg} ->
            Seg
    end.

open_file(Dir, SegConf) ->
    File = case find_segment_files(Dir) of
               [] ->
                   F = ra_lib:zpad_filename("", "segment", 1),
                   filename:join(Dir, F);
               [F | _] ->
                   F
           end,
    %% There is a chance we'll get here without the target directory
    %% existing which could happen after server deletion
    case ra_log_segment:open(File, SegConf#{mode => append}) of
        {ok, Segment} ->
            Segment;
        {error, missing_segment_header} ->
            %% a file was created by the segment header had not been
            %% synced. In this case it is typically safe to just delete
            %% and retry.
            ?WARN("segment_writer: missing header in segment file ~s"
                  "deleting file and retrying recovery", [File]),
            _ = file:delete(File),
            open_file(Dir, SegConf);
        {error, enoent} ->
            ?WARN("segment_writer: failed to open segment file ~s"
                  "error: enoent", [File]),
            enoent;
        Err ->
            %% Any other error should be considered a hard error or else
            %% we'd risk data loss
            ?WARN("segment_writer: failed to open segment files~w"
                  "error: ~W. Exiting", [File, Err, 10]),
            exit(Err)
    end.
